In North America, Canada, North Europe and elsewhere, rock salt is sprayed in winter for preventing roads freezing, and therefore steel sheets used for the components of automobile bodies are required to have excellent corrosion-resisting performance. For this reason, in such a use, there have been recently applied pure zinc-plated steel sheets or zinc alloy-plated steel sheets (for example, Zn-Fe alloy-plated steel sheets, Zn-Ni alloy-plated steel sheets, and so forth) having excellent corrosion resistance.
However, there has been a problem that, in the case of a single-layer coating, these plated steel sheets may bear craterings generated on a coating film when the cationic eletrodeposition coating is carried out after phosphating, to give poor appearance of the coating.
Now, as a steel sheet that has solved the problem of the coating appearance, a double-layer plated steel sheet has been proposed, wherein an Fe coating that can achieve a good electrodeposition coating performance is further applied on a pure Zn or Zn alloy coating. Conventionally known steel sheets of this type may include those wherein an upper layer comprises an Fe-Zn alloy coating having an Fe content of 60 to 90 wt. %, and those wherein an upper layer comprises an Fe coating. It is true that the application of the cationic electrodeposition coating on these double-layer plated steel sheets may result in generation of a decreased number of craterings on a coating film and can improve the coating appearance.
However, in order to lessen the generation of craterings on a coating film by providing the Fe-Zn alloy coating having Fe content of 60 to 90 wt. %, the coating weight must be made not less than 5 g/m.sup.2 (per one side), necessarily resulting in higher production cost. Moreover, this Fe alloy coating is so hard and brittle that an infinite number of cracks may be formed when a plated steel sheet is worked into a component, with the result that the lower layer is exposed at the cracked portion. Therefore, when the electrodeposition coating is carried out, it follows that the electrodeposition coating is directly applied on the lower layer, and also that craterings are liable to be generated on the coating film.
On the other hand, in the case of the Fe coating, which is softer than the Fe-Zn alloy coating, no cracks are generated even when the plated steel sheet is worked into a component, and only a little cratering is generated on the coating film. However, if the coating weight is less than 3 g/m.sup.2 (per one side), there is seen variation in the quantity of the generation of craterings. Although the variation factor has not been made clear, this is presumably because the covering rate in the upper layer coating relative to the lower layer coating is so poor, or the purity of the upper layer coating is so high, that large crystals of phosphate may tend to be formed during phosphating which is a pre-treatment for the electrodeposition coating, and, as a result, the rate of covering by the phosphate crystals on the surface of a coating may be lowered and also the variation in the covering rate may be caused to bring about a difference in the electrolytic conduction for electrodeposition coating, between the phosphate-deposited portion and non-deposited portion. Therefore, in order to lessen the generation of craterings on a coating film by providing the Fe coating, the coating weight must be 3 g/m.sup.2 (per one side), also necessarily resulting in higher production cost.
Taking account of the fact of that the electrodeposition coating performance has not been perfect even in the double-layer plated steel sheet obtained by applying the Fe coating on the pure Zn coating or Zn alloy coating as mentioned above, this invention aims at providing a plated steel sheet that has been improved with regard to the electrodeposition coating performance and yet can achieve lower production cost.